The present invention relates to a wrist carried wireless instrument, and in particular, to an improved antenna resonance circuit for a wrist carried wireless instrument.
Wrist carried wireless instruments are known in the art as exemplified by wrist mounted AM radios, FM radios, FM transceivers and miniature pocket pagers. Each device uses a distinct form of antenna. The miniature AM radios utilize a ferrite antenna stored in the case. The FM radios utilize a wireless antenna which is connected to the case and also serves as an earphone to receive signals. FM transceivers use a rod antenna which is drawn from the wrist carried case to be used during transmission. The pocket pager utilizes a small sized loop antenna having an area of several centimeters square to about 10 centimeters square. The resonance frequency of the antenna may be varied by varying the capacitance.
The above antennas have been satisfactory. However, if the ferrite antenna of the AM radio is to be used in connection with a high frequency such as a VHF band, it becomes necessary to provide a large sized ferrite antenna making it extremely difficult to carry the radio and the antenna on the wrist. Therefore, the use of a ferrite antenna is limited to the traditional AM radio frequencies.
The rod antenna of the FM transceiver and the antenna of the FM radio are also carried on the wrist therefore making it an obstacle to normal activities due to the large size of each antenna. Additionally, if the antenna is to be stored within the device and removed as needed, it becomes difficult to continuously place the antennas in either the receiving or transmitting condition as necessary due to the large size.
The receiving sensitivity of the pocket pager becomes inferior as the loop area of the loop antenna decreases. Accordingly, when the loop antenna is actually small enough to be placed within the case of a wrist carried pocket pager, the sensitivity becomes extremely inferior for the purpose of receiving the relatively large VHF long waveform. Therefore, when the pocket pager is formed as a wrist carried device, the loop antenna is formed outside of the case. To obtain good sensitivity it becomes necessary to have as large a loop area as possible while maintaining a shape which is convenient and fits about the wrist.
For example, a wrist carried loop may fit about the wrist as shown in FIG. 1a, 2a or a loop having a case surrounding shape shown in FIG. 8 may be used. As seen in FIG. 1a, a wireless instrument generally indicated at 30 includes a case for having a loop antenna 10 extending therefrom. Loop antenna 10 includes an opening 17 within the loop. A capacitor C.sub.3 is provided across loop opening 17. An RF amplifier 12 is connected to loop opening 17 between capacitor C.sub.3 and one end of loop antenna 10. RF amplifier 12 connects loop antenna 20 to the remainder of the wireless instrument circuitry contained within case 4. Capacitor C.sub.3 is grounded at its other end.
A wireless instrument 35 constructed in accordance with the prior art as shown in FIG. 2a also contains a resonance circuit. The resonance circuit of instrument 35 is similar to that of instrument 30. However, two capacitors C.sub.4 and C.sub.5 are provided in series across loop antenna opening 17. Capacitors C.sub.4 and C.sub.5 are connected at a terminal 27. RF amplifier 12 connects to the resonating circuit at terminal 27.
These wrist carried wireless instruments having an antenna resonance circuit as shown have been less than satisfactory. They suffer from the disadvantage that if the antenna is brought in contact with the wrist, the wrist provides a load relative to the resonance circuit of the loop antenna. This greatly reduces the quality of the resonance circuit as well as the sensitivity of the instrument making the wrist carried wireless instrument unsuitable for practical use. When the loop antenna does not surround the wrist, but surrounds the case as shown in FIGS. 8 and 9, the circuit quality becomes reduced as the loop antenna approaches or contacts the wrist thereby deteriorating sensitivity.
Accordingly, it is desired to provide a wrist carried wireless instrument which overcomes the above shortcomings and provides a wrist carried wireless instrument which prevents deterioration of quality due to contact with the wrist while providing good sensitivity to receive the signals.